The present invention relates to surface acoustic wave (SAW) devices, and more particularly to a surface acoustic wave lowpass filter.
A SAW filter generally, has a substrate made of piezoelectric material capable of generating a surface acoustic wave, a transducer, formed on the substrate, having an electrode pattern to energize the surface wave, and a transducer to receive the energized surface wave. In order to achieve an excellent filter characteristic, at least one of the transducers is usually composed in an apodized electrode pattern. The configuration of the apodized electrode pattern is determined by the impulse response of a finite impulse response (FIR) filter having the filter response. The impulse response is assessed to determine the amount of overlap between the electrode fingers of the apodized transducer. The amount of overlap between the fingers is proportional to the sampling values (tap coefficients) of the impulse response of the FIR filter. The tap coefficients of the FIR filter are also known as weighted coefficients of the SAW filter. As high-frequency electric signals are applied to the transducer having the apodized electrode pattern, a surface acoustic wave is excited, an impulse response is obtained from the other transducer as its output, and thereby the input-output response presents the frequency characteristic of the FIR filter.
Although the aforementioned type of apodized transducer is used in bandpass filters as described in D. W. Parker et al., "Acoustic surface-wave bandpass filters", Philips Technical Review, 1976, volume 36, No. 2, pp. 29-43, its use in lowpass filters has not yet been studied well. In a system for transmitting audio and video signals, a lowpass filter having a steep frequency characteristic in the video pass band of 4 to 5 MHz is usually required to separate video signals, and accordingly realization of a SAW lowpass filter giving a comparatively steep filter characteristic is hoped for.